1. Field of the Invention
The present invention relates generally to an acoustic combination responsive to a sound wave, and more particularly, to an acoustic energy reclamation device that extracts energy from the sound wave and an acoustic liner that has an adjustable compliance that can be adjusted to attenuate the sound wave.
2. Background
Passive Liner Technology
Sound-absorbing acoustic panels have been widely utilized in turbofan engines for noise suppression of engine duct noise. These acoustic panels line the engine duct surface and provide an impedance boundary condition for the acoustic modes propagating within the duct. A typical single degree-of-freedom (SDOF) acoustic liner or Helmholtz resonator 50, shown in FIG. 1(A), is composed of a face sheet 10 and honeycomb core 12 with a rigid backing sheet 14. FIG. 1(B) shows an alternative embodiment where Helmholtz resonator 100 is composed of a face sheet 20 and honeycomb core 22 with a rigid backing sheet 24. Face sheets are usually composed of a perforated plate 10 as shown in FIG. 1(A) or woven wire/perforated plate sandwich 20 as shown in FIG. 1(B). The perforated plate face sheet 10 has a flow resistance controlled by percent open area (i.e., number of holes and hole size) and face sheet thickness. Likewise, the woven wire/perforated plate sandwich 20 has a flow resistance (Rayl number) controlled by percent open area (i.e., number of holes and hole size), face sheet thickness, wire size and wire density. The honeycomb core 12 or 22 is composed of cells 16, 26, respectively, which, when bonded to the face sheet 10 or 20, create cavities behind the face sheet 10 or 20. The attachment of an impervious backing sheet 14 or 24 to the honeycomb core 12 or 22, respectively, seals the honeycomb core 12 or 22 so that each cavity is isolated from its neighbors, thereby creating a “locally reactive” liner. The impedance of a conventional, passive SDOF liner 50 or 100 in a given acoustic medium is a function of the device geometry and grazing flow conditions. The effective frequency range of existing passive SDOF acoustic liners is limited to one octave. Typically, these panels are tuned to the turbofan blade-passage frequency of interest.
Multiple degree-of-freedom (MDOF) systems, such as a double-layer liner 200 that is composed of a face sheet 120 and porous septum 122 with a rigid backing sheet 124 as shown in FIG. 2, and bulk (or “globally” reactive) absorbers offer a wider suppression bandwidth (2–3 octaves), but represent a tradeoff in terms of design complexity, structural integrity, size, weight, and cost. As is the case with SDOF liners, the impedance of MDOF liners is also a function of the device geometry and grazing flow conditions. Indeed, bulk-absorber materials do exist that exhibit desirable acoustic characteristics, although none were deemed usable in aircraft engines. This is because these materials showed a strong tendency to absorb hydrocarbons such as jet fuel and hydraulic fluid in fluid absorption tests.
The greatest limitation of passive liner technology is the constraint of fixed impedance for a given geometry. For a given aircraft propulsion system, there will be different optimum nacelle impedance distributions for the differing mean-flow and acoustic source conditions associated with take-off, cut-back, and landing conditions. Existing active liner technology offers the promise of in-situ adjustable liner impedance, but has the associated drawbacks in terms of cost, complexity, and weight.
Active Liner Technology
Active acoustic liners have been studied recently because of their potential to enhance the performance of the passive liners described above. A review of existing technology in this area is briefly summarized here, in which steady bias flow and/or variable-volume Helmholtz resonators are used to increase the effective suppression bandwidth of the liner.
One such study is described in De Bedout, J. M., Franchek, M. A., Bernhard, R. J., and Mongeau, L., “Adaptive-Passive Noise Control With Self-Tuning Helmholtz Resonators,” J. Sound and Vibration, vol. 202(1), pp. 109–123, 1997, in which a tunable, variable-volume Helmholtz resonator is combined with a robust, simple control algorithm to achieve maximum noise suppression. The robust control algorithm developed for tuning the resonator is a combination of open-loop control for coarse tuning with closed-loop control for precise tuning. The coarse tuning adjusts the resonator volume based on a lumped parameter model, while the precise tuning algorithm uses a gradient-descent-based method to minimize the voltage output of the microphone. One disadvantage of the approach of De Bedout et al. is the difficulty associated with the mechanical implementation of variable-volume resonator (via a sliding wall) in an acoustic liner.
Howe, in “On the Theory of Unsteady High Reynolds Number Flow Through a Circular Cylinder,” Proc. Royal Society of London A, vol. 366, pp. 205–223, 1979, theoretically modeled the Rayleigh conductivity of circular apertures in thin plates in the presence of mean bias flow through the holes. His work represented an extension of the work of Leppington and Levine as described in “Reflexion and Transmission at a Plane Screen with Periodically Arranged Circular or Elliptical Apertures,” J. Fluid Mech., vol. 61, pp. 109–127, 1973, who examined the problem of reflection of sound by a rigid screen perforated by an array of circular or elliptical apertures. In Howe's model, the incident sound interacts with the mean bias flow to produce vorticity fluctuations, the magnitude of which is determined by the Kutta condition at the edge of the aperture to avoid a velocity singularity. The significance of Howe's work is that it showed the promise for noise attenuation via a small amount of mean bias flow through the apertures of an acoustic liner. Hughes and Dowling in “The Absorption of Sound by Perforated Linings,” J. Fluid Mech., vol. 218, pp. 299–335, 1990, verified this concept via a series of experiments in a normal impedance tube.
Sun and his colleagues have conducted further experimental studies of perforated liners with bias flow as shown in “Experimental Investigations of Perforated Liners with Bias Flow,” J. Acoust. Soc. Am., vol. 106(5), pp. 2436–2441, November 1999, and “Active Control of Wall Acoustic Impedance,” AIAA J., 37, No. 7, 825–831, 1999. They found that a bias flow could markedly increase both the absorption coefficient and effective bandwidth of a perforated liner. The improvement is presumably due to the fact that the bias flow increases the acoustic resistance, although the change in the acoustic reactance is slight. Plate thickness is shown to have a major impact on the performance of the liner, changing the reactance and, hence, the natural frequency of the liner. Reasonable agreement is obtained between experimental data and theoretical values derived from the theory of Howe, adapted to account for finite plate thickness. They have also developed a feedback control system to vary liner cavity depth and bias flow rate in order to optimize the absorption coefficient or maintain the desired impedance in a normal impedance tube, independent of sound frequency. Note that their variable-depth cavity is essentially the same as the variable-volume resonator in De Bedout et al. (1997) and therefore has the same disadvantage mentioned above. It is also worth noting that the authors emphasize the need to find a practical way to vary the reactance of the liner in a real application (Zhao & Sun, 1999).
Walker et al. in “Active Resonators for Control of Multiple Spinning Modes in an Axial Flow Fan Inlet,” AIAA paper 99–1853, 1999 demonstrated an active Helmholtz resonator with an improved absorption bandwidth by adding a controlled volume velocity via a secondary sound source. This was realized by driving a flexible backplate actuator as part of a feedback control system. While a promising technique, this configuration like all active systems as described above requires actuators, sensors, and a feedback controller. Each of these key components requires power and must be linked via a communication system, typically entailing electrical wiring. Depending on the actuation, sensing, and wiring schemes, such a distributed system is often complex and potentially expensive to implement from a power consumption standpoint.
Thus, there is a need to develop a self-powered, wireless, acoustic liner technology with the performance of an active system, yet with the simplicity and reliability of a passive system.